The present invention relates to a method and a device to allow a rigid pig to pass into a flow pipe which requires the use of a hollow flow-constricting device. The proposed method and device are particularly suited to be used in an undersea flow pipe.
Pipes are widely used in the industry to transport diverse kinds of fluids. Such fluids may comprise a single constituent or multiple constituents, they may comprise a single phase or multiple phases, they may be highly compressible or they may be almost incompressible. Such pipes may be provided with varying internal diameters and configurations. In the oil industry the pipes, or flow pipes as they are usually referred to, are used to promote the flow of fluids from oil producing wells to gathering centres where the fluids are processed.
When an undersea oil field is commercially exploited, it is necessary for the production from the oil producing wells to flow through pipes to a production unit located at the surface. An undersea flow pipe is usually connected at one end to an oil producing well and extends along the sea bed to be connected at its other end to an undersea flow riser, which carries the fluids to the production unit at the surface.
Situations may occur in which it is necessary to insert into the undersea flow pipe at a certain location a hollow flow-constricting device which constricts the flow at that location. Such device can be, for example, a body externally shaped to match the inside portion of the undersea flow pipe where it is to be located, and have an orifice of any shape extending longitudinally therethrough so as to provoke a constriction in the flow.
The object of the insertion of such a device into the undersea flow pipe may be, for example, (a) to introduce a constriction in the flow to control the features of this flow, or (b) to introduce an element intended to be used in flow rate measuring operations, or (c) to control the phenomenon known as severe slugging which may occur in production systems provided with descending flow pipes followed by flow risers.
The severe slugging phenomenon is characterised by intense oscillations in the pressure and flow rate levels occurring in a multiphase flow having a gaseous phase. The severe slugging phenomenon causes difficulties in undersea production activities, which can seriously impair or even shut the oil production down.
The patent application GB 2 341 695, commonly owned by the applicants of the present patent application, discloses a device used to control the severe slugging phenomenon. A hollow flow-constricting device, preferably a venturi, is installed into a descending undersea flow pipe relatively close to the junction to a flow riser.
The design of a new undersea flow pipe may anticipate the need of such hollow flow-constricting device, which can be installed during the deployment of the undersea flow pipe. In existing undersea flow pipes, where access to the interior of the undersea flow pipe is easy and the oil production flow can be interrupted, the hollow flow-constricting device can be installed after cutting the undersea flow pipe, the integrity of the undersea flow pipe being reinstated by using any of the known pipe assembling techniques, e.g., welding.
However, besides causing ceasing of profits, shutting down of the production can cause many operational problems, especially in undersea flow pipes under the effects of low temperatures of the sea bed. Many of these undersea flow pipes are located at great sea depths, hindering the access by divers. A cutting operation in such an undersea flow pipe would be very difficult to be implemented, as it involves the retrieval of the undersea flow pipe from the seabed, or carrying out the operation using a remote operated vehicle, both being extremely expensive, time consuming and complex operations.
Thus, there is a need to provide a way of installing in an undersea flow pipe a hollow flow constricting device such as is described in GB 2 341 695, without causing the above drawbacks.
The British patent application 0102331.6, of Jan. 30, 2001, commonly owned by the applicants of the present patent application, discloses a method of setting a hollow flow-constricting device into a submarine flow pipe using flexible rigid pig, preferably foam rigid pig formed from polymeric foam which may be radially compressed when passing though a constriction. The word xe2x80x9cpigxe2x80x9d is used here to denote devices which are inserted into a pipe and which are urged by the flow flowing into that pipe, usually to clean the interior of the pipe.
Situations may occur in which it is needed for any reason to carry out an operation of passing into the undersea flow pipe a rigid pig which is not entirely compressible. For example, it is usual to pass a rigid pig into a flow pipe for inspecting its internal walls. Such a rigid pig includes a number of rigid bodies encasing inspecting equipment, said bodies being longitudinally connected between them by means of flexible unions, whereby the rigid pig is relatively longitudinally flexible, in order to pass through the curves of the flow pipe.
Usually such bodies, or most of them, are provided with external packing elements which are radially mounted to the bodies. Said packing elements are able to be compressed between the internal walls of the flow pipe and the external walls of its respective body, thereby forming a seal. Therefore, this kind of rigid pig is able to be partially radially compressed, but only to the radial extent of the packing elements. In other words, as the bodies are radially rigid, such a rigid pig may be partially longitudinally curved, but it is unable to be substantially radially compressed.
Therefore, such a radially rigid pig is not able to pass through the hollow flow-constricting device, which would preclude the above pig operation from occurring.
It should be mentioned that the retrieval of such a hollow flow-constricting device before each pigging operation although feasible is much too expensive and is not recommended.
The present invention proposes a novel device and method which overcome the above mentioned drawbacks, and can enable a rigid pig (as hereinafter defined) to be passed into an undersea pipe in which such a hollow flow-constricting device is used.
The present invention relates to a device and a method to allow a pig to pass into a flow pipe which requires the use of a hollow flow-constricting device.
In a first aspect the present invention encompasses a device to allow a pig to pass into a flow pipe in which a hollow flow-constricting device is in use, said device comprising:
a first pipe branch, provided with an upstream end and a downstream end;
a second pipe branch, provided with an upstream end and a downstream end;
a hollow flow-constricting device installed in the first pipe branch;
a first shut-off valve, installed in the first pipe branch;
a clear-flow second shut-off valve, installed in the second pipe branch;
a grating device, installed at the upstream ends of the first pipe branch and the second pipe branch to prevent the rigid pig from entering the first pipe branch, and
wherein:
the upstream ends of the first and second pipe branches are interconnected at an upstream junction which is provided with an upstream connection; and
the downstream ends of the first and second pipe branches are interconnected at a downstream junction which is provided with a downstream connection.
In a second aspect the present invention comprises a method of using the device of the first aspect to allow a pig to pass into a flow pipe in which a hollow flow-constricting device is in use, said method comprising the steps ofxe2x80x94
initially opening the clear-flow second shut-off valve, which is kept closed under normal operational conditions, whereby the flow is led to pass also into the second pipe branch;
next closing the first shut-off valve, which is kept open, under normal operational conditions, thereby blocking off the flow from passing into the first pipe branch, whereby the flow is led then to pass only into the second pipe branch;
next inserting a pig into a mechanism suited for such operation, located at any region of the upstream flow pipe segment;
allowing the pig to travel along the upstream flow pipe segment, urged by the flow, to pass into the upstream connection, next to travel along the second pipe branch to pass through the clear-flow second shut-off valve, and then to pass into the downstream connection and to keep on travelling along the downstream segment of the flow pipe;
then opening the first shut-off valve, after the pig has passed the downstream connection; and
next closing the clear-flow second shut-off valve, thereby reinstating the normal operational conditions.